1. Field of the Invention
The present invention relates to a process for producing granules for molding ferrite capable of obtaining a ferrite green body excelling in molding properties, the granules for molding ferrite capable of obtaining a ferrite green body obtainable from the same, a ferrite green body and a ferrite sintered body.
2. Description of the Related Arts
Ferrite has been widely utilized in various fields including electronic parts. The ferrite is obtainable by granulating raw ferrite powder together with a binder into ferrite granule to obtain a ferrite green body and sintering the resulting ferrite green body.
Conventionally, various processes have been applied to produce a ferrite green body. Amongst them, a dry molding process under a pressure has generally been performed.
For example, a ferrite green body is produced by preparing a ferrite slurry from raw ferrite powders and water, spray-drying the resulting slurry through a spray dryer to produce granules for molding ferrite, which is formed under a pressure.
Alternatively, ferrite granules for molding are granulated by stirring and mixing the raw ferrite powders and the binders, and the repeating drying and oscillating extrusion. The term “oscillating extrusion” used herein is a process in which, for example, particles granulated for example with several mm are crushed on a net, and finely divided particles are dropped, these operations are repeated with using nets with finer mesh in several stages whereby particles having a prescribed size are obtained.
The granules for molding ferrite (hereinafter referred to as “ferrite granules”) are required to have the following properties in order to produce a ferrite green body.    (1) The ferrite granules should have flowability within a suitable range, and a good packing property during the course of packing the ferrite granules into a mold in a uniform manner.    (2) The ferrite granules should be crushed at a low pressure (typically from 29 to 147 MPa) during the course molding (hereinafter referred to as “a crushing property at a low pressure”).    (3) Ingredients such as fine particles contained in the ferrite granules should not be stuck to a mold or the like (hereinafter referred to as “anti-sticking properties”).    (4) The ferrite granules are not collapsed during the course of the storage, transportation, agitation when they are packed into a mold, or due to collision with each other (hereinafter referred to as “an anti-collapse property”).    (5) The ferrite granules have suitably bulk density so that they flow out of the mold during the course of packing them into a mold, or of molding (hereinafter referred to as “a mold-packing property”)
In order to satisfy these requirements, various processes for granulating the ferrite granules have been suggested. For example, Japanese Patent Laid-Open Publication No 4-137704 discloses a process in which ferrite granules are granulated through an atomizer at a granulation temperature of from 270 to 290° C. to adjust the water content of from 0.5 to 0.7% and Japanese Patent Laid-Open No 10-38460 discloses a process in which a binder is sprayed onto granulating powders within a sprayer dryer.
Japanese Patent Laid-Open No. 2000-272970 discloses a process utilizing a polyvinyl alcohol polymer component containing α-olefin with up to 4 carbon atoms or containing a vinyl ether as a binder.
However, these prior techniques, which can improve the strength of the ferrite green body, are disadvantageous in that the resulting ferrite granules are rigid, has a poor crushing property at a low pressure, and many boundary deficiencies remain in the resulting ferrite green body. Consequently, these prior arts do not give any satisfactory effects in terms of strength and dielectric characteristics of the ferrite sintered body.
Also, these techniques have the following drawbacks.    a. These techniques relate to improvement of ferrite slurry, the granulation process applicable to these techniques is restricted to a spray-drying granulation process using a spray dryer and, thus, these techniques are of low flexibility.    b. Although the flowability and the crushing property at a low pressure of the ferrite granules are somewhat improved, the something to further improved has been still left. Also, the molded product produced by the use of such ferrite granules has insufficient dimensional accuracy. Further, these ferrite granules are not suitable for producing a ferrite product having a complicated shape.    c. The ferrite granules are sometimes collapsed during the course of the storage, transportation, or packing them into a mold.    d. When being molded in a mold, the fine ferrite particles are stuck to the mold, causing sticking, which in some cases makes it impossible to continuously produce ferrite green bodies.    e. The resulting ferrite green body is poor in the mechanical strength and there is tendency that defective such as chipping or breaking will easily occur.    f. The dielectric characteristics of the resulting ferrite sintered body, particularly magnetic loss cannot be obtained in a sufficient manner.            g. The ferrite green body resulting from the granules only have a poor mold release property so that crack tends to occur in the interior of the ferrite green body due to the spring back.        
The term spring back (swelling of volume” used herein the volume of ferrite green body is swelled during the course of subtracting the ferrite green body from a mold to be larger than the size of the mold.